Many industrial facilities have piping systems and assemblies handling fluids that are corrosive and/or erosive to conventional piping materials of construction such as steel. In some piping systems, more costly corrosion-resistant materials of construction are used for the pipe assemblies handling these fluids. In other applications, the initial wall thickness of a steel piping system is increased to allow for a loss of thickness from corrosion and erosion without compromising the safe handling of the fluid within the piping system. Periodic inspections of conventional and corrosion-resistant piping systems are typically accomplished at these and other facilities to ensure that the remaining wall thickness is at least up to the minimum standards for safety.
In some corrosive/erosive applications, the handled fluids or fluid-like materials are hot, typically requiring the exterior of the piping to be insulated. Visual or other pipe inspection methods requiring access to or contact with the pipe wall must therefore include the removal of the exterior insulation or the use of inspection devices placed within the pipe. Removal of all insulation may not only be costly, but may damage the insulation. If pipe inspections are conducted at specific points, small plugs of insulation are typically removed to allow access to these points on the pipe. However, these point inspection methods may fail to detect substandard pipeline conditions that occur beyond the inspection points.
A hot corrosive fluid contained within the piping system may essentially preclude the use of some inspection devices placed within the pipe. If the pipe interior is lined, interior visual or contact inspection of the piping may also be precluded. Even if not precluded, interior inspections of piping may require costly process or facility shutdowns.
Several electromagnetic (EM) testing techniques have been employed which do not require direct pipe access or contact. These include magnetic flux methods and eddy current devices. In one type of magnetic flux method as described in U.S. Pat. No. 4,555,665, a coil is wrapped around a pipe and a saturating DC current is supplied to the coil while the pipe-disturbed magnetic flux in the proximity of the pipe is detected and correlated to average wall thickness.
An intermediate- or high-frequency EM signal source is typically used in an eddy current device to induce eddy currents in the nearest surface of a pipe and the detected eddy currents are correlated to defects at that surface. Several intermediate- or high-frequencies may be used, for example as described in U.S. Pat. No. 4,467,281, frequencies ranging from 25 kHz to 1.6 MHz. In some eddy current devices, low-frequency signals are filtered out to provide better resolution of the intermediate or high frequency signals representative of surface or near-surface eddy currents and the impacts of surface defects on these eddy currents.
Both the magnetic flux and eddy current methods require relatively close proximity of EM coils and/or detectors to the pipe. At further distances from the pipe, the magnetic flux distortion caused by the pipe will not be detectable and thus the thickness of the pipe will be unmeasured by the magnetic flux method. Also, at larger distances from the pipe, the intermediate or high frequency signal of an eddy current device will be insufficient to generate detectable eddy currents in the surface of the pipe to be inspected. In addition to the proximity requirement, the presence of intervening conductive and/or non-conductive materials will adversely impact these methods. For example, if used on insulated pipe covered by a metal jacket, the eddy current device would detect defects in the outside jacket surface instead of the outside pipe surface.